1. Field of the invention
The present invention relates to a multilayer interconnection and method for manufacturing the multilayer interconnection. More specifically to a multilayer interconnection formed of oxide superconductor material and method for manufacturing the same.
2. Description of Related Art
A superconducting current path is one of the electronic applications of a superconductor. If all the current paths of a conventional electronic circuit including semiconductor devices is replaced with superconducting current paths, completely, the electronic circuit will operate rapidly with low power consumption. Superconducting signal paths are also expected to reduce the wave form distortion so that the required number of amplifiers and/or repeaters can be reduced. Particularly, by using an oxide superconductor material which has been recently advanced in study, it is possible to produce a superconducting current path through which superconducting current flows at relatively high temperature.
An oxide superconductor has the largest critical current density J.sub.c in direction perpendicular to c-axes of its crystal lattices. Therefore, it is desirable that the superconducting current path through which superconducting current flows horizontally is formed of a c-axis orientated oxide superconductor thin film and the superconducting path through which superconducting current flows vertically is formed of oxide superconductor thin films of which c-axes are orientated horizontally. In this specification, this oxide superconductor thin film of which c-axes are orientated horizontally will be called an "a-axis orientated oxide superconductor thin film".
In a prior art, a superconducting multilayer interconnection which has layered superconducting current paths constituted of c-axis orientated oxide superconductor thin films and a superconducting interconnect current path constituted of an a-axis orientated oxide superconductor thin film is proposed. In the superconducting multilayer interconnection, the superconducting interconnect current path is formed at a via hole penetrating through the layered superconducting current paths. The via hole increases the occupied area of the superconducting multilayer interconnection.
In addition, grain boundaries are generated at the interface between the c-axis orientated oxide superconductor thin film and the a-axis orientated oxide superconductor thin film, which introduce difficulties of superconducting current flowing. The grain boundaries sometimes form Josephson junctions which pass only tunnel current so that the current capability is limited and the input and output characteristics become nonlinear. Even if no Josephson junction is formed at the interface, Joule heat may be generated by the electrical resistance formed at the interface, which causes the "quench" phenomenon. Furthermore, the c-axis orientated oxide superconductor thin film and the a-axis orientated oxide superconductor thin film may interfere with each other so as to degrade each other.
In order to manufacture the above superconducting multilayer interconnection, in a prior art, the via hole is formed through layered superconducting current paths and insulating layers and an a-axis orientated oxide superconductor thin film is formed in the via hole. The diameter of the via hole is made small in order to decrease the occupied area of the superconducting multilayer interconnection. Therefore, the side surface of the via hole can not be cleaned enough, which generates an unnecessary Josephson junction when the a-axis orientated oxide superconductor thin film is formed.